Overcoming the Ambiguity of Physiological Adaptation

The human body is an extremely complex organism, and our physiology’s ability to respond and adapt to a variety of stressors (in order to withstand future stressors more efficiently) is a fascinating example of such complexity. However, our bodies can respond to eustress and distress very similarly meaning we cannot, without context, distinguish between a positive and negative adaptation. For example, an increase in the heart’s left ventricular mass could be explained by consistent endurance training or heart disease. How can the same physiological adaptation be a sign of both positive and negative health, and how can we best navigate this problem to best inform endurance training practise?

Grigne SkyMarathon. Image from The Pill.

The two primary models to explain how the body responds to different stressors are Selye’s general adaptation syndrome model and Yakovlev’s theory of super compensation.

Selye’s model aims to showcase the physiological consequences of distress in three main stages: alarm, resistance and exhaustion. The alarm phase describes that when a human is met with a stressor, it invokes a rise in both cortisol and epinephrine, the physiological responses of which include an increased heart rate, blood pressure and even a release of glycogen from the liver. Our palaeolithic selves would find these responses extremely valuable as they would play a role in allowing us to either escape predators or catch prey, but our modern day lives are full of stressors that may trigger these responses. Being late for a meeting, arguing with a family member or - for those with social anxiety - even speaking to new people may produce these responses. In the short term, these responses aren’t harmful, but consistent distress can play a role in these physiological responses (heart rate or blood pressure) being elevated for chronic periods, which is termed the resistance phase. There’s only a certain amount of time in which these responses can be elevated before they become harmful. The exhaustion phase is when that time comes to an end and a chronic elevation of these physiological responses results in negative outcomes, like disease. Directly, constantly elevated blood pressure may result in hypertension. Indirectly, Levins’ principle of allocation explains that allocating energy to one function reduces the energy available for other functions. An over reliance on one function (used to combat a daily stressor in one’s life) may reduce function of another area, making that area more vulnerable to harm.

Yakovlev’s theory of super compensation explains how humans tend to respond and adapt to exercise. The theory suggests that following exercise (in this case, the stressor, albeit eustress), the human body enters a state of fatigue that allows for “baseline” performance to be replicated. However, following the recovery from that bout of exercise, the athlete is able to achieve a level of performance above their original baseline, which can be attributed to a physiological response (in the case of endurance sport, this could be an increased mitochondrial or capillary density, ventricular mass or monocarboxylate transporters, for example). The human body responds not to a single bout of exercise, but repeated bouts over an extended period, so the process of super compensation can be most accurately explained through a marathon training block: due to numerous weeks of large training volumes (often averaging more than 14 miles/23 kilometres per day or 100 miles/162 kilometres per week), the athlete is unlikely to be able to achieve their “fresh state” marathon performance until they reach a period of de-loading (a taper) which allows for an absorption of previous training volume, for fatigue to clear and adaptations to be made. Following the taper, the athlete reaches marathon race day in a fresh state with enhanced performance due to the adaptations made both during and following the previous training period. 

Short term, the body responds to eustress and distress in slightly different ways but seems to adapt to that stress through physiological changes that, on the surface, seem identical:

The formation of new mitochondria, termed mitochondrial biogenesis, can occur following prolonged eustress and distress. In the locomotor muscle, an increase of mitochondria can occur following consistent endurance training and is often associated with improved performance due to improved oxygen utilisation. However, mitochondrial biogenesis has been found to occur in the brains of mice following 72 hours of sleep deprivation, an adaptation that likely occurs to balance out the numerous negative metabolic, cardiovascular and pulmonary disruptions associated with both acute and chronic lack of sleep.

The upregulation of heat shock proteins is another mechanism that responds to both positive and negative stressors. An increase in HSP’s could be due to successful heat adaptation in preparation for an event in hot weather or could be in response to neurodegenerative disease. 

An increase in red blood cell mass could be explained by altitude acclimatisation, which would enhance the body’s ability to carry oxygen to the periphery. It can also occur in patients with polycythemia which can pose an increased risk for blood clots.

Olympic athletes likely possess an above average left ventricle volume, allowing for improved stroke volume. However, this adaptation is shared with sufferers of pathological cardiac hypertrophy, who are at an increased risk of sudden cardiac death.

The overarching point is that without context, and the ability to view the body as a whole, determining if an adaptation is positive or negative is complete guesswork, and the same idea can be applied to endurance sport. We cannot be sure that an athlete will become faster because their mitochondrial density has increased and we cannot be sure that the athlete is less fit because of reductions in haemoglobin. A true embrace of the body’s complexity is to utilise the information we are certain of and to refrain from assumptions. For now, due to the ambiguity related to physiological adaptations and the lack of suitable technology, a simple approach to performance evaluation is likely the most effective approach. An input-output system, while technology is still in its infancy, is most suitable. An input measure is one of internal load (heart rate, blood lactate or effort perception, for example) and an output measure is one of external load (velocity or power). Everything in between (which aims to explain the “why” behind changes in input and output) we cannot be certain of, but does that really matter if the internal load is decreasing or the external load is increasing? As sports scientists, coaches and athletes it’s important to remember our true goal is not to produce the least blood lactate, but to be the fastest athlete. 

Despite using blood lactate and heart rate as example measures of internal load, I still believe that those who exclusively use these measures are falling into a reductionist trap. Our perception of effort is the most holistic measure of exercise intensity and performance improvement as it is a number generated from a vast number of internal datapoints. Manuel Sola Arjona, cycling coach and one of endurance sport’s best modern thinkers, put it best: 

“Perception of effort is a high capacity processor that can monitor in real time and keep a history of the stress data of each part of the organism, weighing them in importance according to how much they affect us at any given moment and taking into account chances in the environment, the type of task and the importance of its success for our survival. In addition, this processor has a machine learning layer: it learns from every action.”

In summary, the body is too complex for us to make assumptions regarding how the body responds to stress. It’s important to rely on data we know to be accurate, which for the time being is a simple input-output system that (ideally) relies on effort perception as the input measure and velocity or power as the output. Understanding the body’s ambiguity is key to training smarter, not harder which is something I help endurance athletes achieve every day through evidence-based coaching that respects the complexity of the human body.